1. Field of the Invention
The present invention relates to a hologram recording method and a hologram recording apparatus, particularly to the hologram recording method and the hologram recording apparatus in which hologram recording is performed with high density.
2. Description of the Related Art
Holographic memory receives attention as a computer memory of the next generation. The holographic memory has both large capacity derived from a three-dimensional recording region and high speed derived from a two-dimensional batch recording and reproducing method.
In the holographic memory, a plurality of data pages can be recorded by multiplexing the data pages in the same volume, and the data can be read out in each page. The digital data can be recorded and reproduced in such a manner that not an analog image but binary digital data “0,1” is digitized in the form of “bright, dark” and recorded and reproduced in the form of the hologram.
In recent years, various proposals on a specific optical system of this digital holographic memory system, an S/N ratio or a bit error rate evaluation based on a volume multiplex recording method, or two-dimensional coding have been made. Also, studies have been made from a more optical engineering point of view such as influence of aberration of the optical system.
Referring to FIG. 12, a shift multiplex recording method which is one of the volume multiplex recording methods will be described (D. Psaltis, M. Levene, A. Pu, G. Barbastathis and K. Curtis; Opt. Lett. 20 (1995) p782). In the shift multiplex recording method, a signal light beam 131 and a spherical wave as a reference light beam 132 are simultaneously irradiated on a hologram recording medium 135, and the hologram recording medium 135 is formed in a shape of a disk, and a plurality of holograms are written over another in the same region by rotating the disk 135. For example, when a beam diameter is set to 1.5 mm, another hologram can be recorded in the substantially same region without generating crosstalk only by moving the disk 135 by several tens μm. This recording is based on a fact that, since the reference light beam 132 is the spherical wave, the movement of the disk 135 is equivalent to a change in an angle of the reference light beam 132.
As shown in the above reference, a moving distance of the spherical reference light wave shift multiplex recording, i.e. a distance δ in which the holograms can be independently separated from each other is given by the following formula (1).δspherical=δBragg+δNA≈(λzo/L tan θs)+λ/2(NA)  (1)
Where λ is a wavelength of the signal light beam, zo is the distance between an objective lens forming the spherical reference wave and a recording medium, L is a film thickness of the recording medium, θs is a crossed axes angle between the signal light beam and the spherical reference wave, and NA is a numerical aperture of the objective lens. As can be seen from the formula (1), the amount of shift δ is decreased as the film thickness L of the recording medium is increased, so that a degree of multiplexity can be increased and recording capacity can be increased.
In order to more efficiently increase the recording capacity in the shift multiplex recording method, the recording region is micronized. The volume multiplex recording having higher density can be realized by performing the multiplex recording in a microregion. For this purpose, the signal light beam is Fourier-transformed by a lens to irradiate the recording medium in the holographic memory system. Accordingly, in the case where an image of the signal light beam has a fine pitch (high spatial frequency), Fraunhofer diffraction occurs on a surface of the recording medium in the signal light beam, and spread ζ of its diffraction image is shown by the following formula (2).ζ=kλfωx  (2)Where k is a constant of proportionality, λ is the wavelength of the signal light beam, f is a focal distance of the lens for Fourier transformation, and ωx is the spatial frequency of the signal light beam.
When the lens having the small focal distance f is used as the lens for Fourier transformation, the recording region can be micronized. This is also shown in chapter 7 of “Holography” (The Institute of Electronics, Information and Communication Engineers). The applicant proposed a technique in which the recording region is decreased by recording, of the Fourier transform image, only the minimum Fourier transform component essentially required for data reproduction (Japanese Patent Application Laid-Open (JP-A) No. 2000-66565).
A phase correlation multiplexing method in which the shift multiplex recording is performed by arranging a random phase mask in an optical path of the reference light beam is known (see the specification of U.S. Pat. No. 5,719,691). In this method, Bragg condition of the recorded hologram is not considered, and complexity of a wavefront of the reference light beam is utilized. The different holograms can be multiplex-recorded in such a manner that the reference light beam having an extremely small auto-correlation function of the wavefront is utilized and the recording medium is shifted by the microamount (up to 10 μm). That is to say, the increase in the recording capacity can be realized.
A technique in which the random phase reference light beam used in the phase correlation multiplexing is generated by a holographic optical element (JP-A No. 2001-60394), or a skip-sort multiplexing method (JP-A No. 2002-40908) has been also proposed.
However, as shown in FIG. 1, the Fourier transform image has the infinite spread at a focal plane. Therefore, in the case where information of the signal light beam is recorded in the form of the hologram, a region larger than the region irradiated with the signal light beam has generally to be irradiated with the reference light beam so that the information is not lost. For example, in the case where the recording medium is irradiated using the spherical wave as the reference light beam in order to perform the shift multiplex recording, the circular region larger than the region exposed by the signal light beam is to be irradiated with the reference light beam. In the phase correlation multiplexing method, the irradiation with the reference light beam having the similar region relative to the signal light beam is performed.
However, in this case, the recording medium is also exposed in a region other than the region where the hologram is recorded, i.e. a region other than the region which has been irradiated with the signal light beam. The new hologram can not be recorded in such an excessively or unnecessarily exposed region and, as a result, there is generated the problem that the recording capacity is decreased.
In the shift multiplex recording method in which the spherical reference wave is utilized, there also arises a problem as described below. In the multiplex recording method, a reference light beam having a wavefront of a steep curvature is generated by using the objective lens having the large numerical aperture (NA). The degree of the multiplexity is increased by using this reference light beam for the recording. As a result, high-density recording is realized. Since the objective lens having high NA spreads the beam in a broad angle, when the reference light beam irradiating area is decreased, it is necessary that the objective lens approaches the recording medium.
As can be seen from the above formula (1), in order to increase the degree of the multiplexity, it is necessary to decrease NA of the objective lens and the distance L between the objective lens and the recording medium. However, when the objective lens approaches the recording medium, there arises a problem that the objective lens interferes with the optical path of the signal light beam to cause loss of some of the information of the signal light beam.